Photolithography processes are used in semiconductor manufacturing to transfer a pattern from a photomask to the surface of a wafer or substrate. As part of a typical photolithography process, photoresist layer (usually a polymeric material that changes properties when exposed to light) is applied over an intermediate structure. The desired pattern is projected onto the photoresist through the photomask and though a series of lenses in an optical projection system. The lenses reduce the scale of the projected image. The reduction provided by the lens can vary depending on the design rules. A typical reduction is on the order of 4×–5× magnification, for example. When the mask pattern is projected onto the photoresist layer on the wafer, the exposed regions become more or less acidic. After the photoresist is developed, portions of the photoresist are removed to provide a patterned photoresist layer.
The ability to project a clear and precise pattern of the smallest features onto the wafer is often limited by the wavelength of the light used. Current photolithography systems use deep ultraviolet (DUV) light with wavelengths of about 248 nm and about 193 nm, which typically can provide feature sizes on the order of about 130–90 nm, for example. To extend 193 nm photolithography to feature sizes of 45 nm and smaller, liquid immersion photolithography techniques are being used. This enables the use of optics with numerical apertures exceeding 1.0.
FIG. 1 is a simplified schematic of an immersion optical projection system 10 of the prior art being used in a photolithography process. The system 10 shown in FIG. 1 is sometimes referred to as a shower configuration. Often in such systems, the fluid 12 is continually circulated to eliminate thermally-induced distortions. In the system 10 shown in FIG. 1, the circulated fluid 12 is located between a last lens element 22 and the wafer 24 during a photolithography process. In the immersion head 26 for the system 10 of FIG. 1, a fluid inlet 14 routes the fluid flow to the space between the last lens element 22 and the wafer 24, and a fluid outlet 16 receives the fluid flow. A typical wafer chuck 28 is shown in FIG. 1, which in this case retains the wafer 24 using a vacuum force provided by vacuum channels 30 and a vacuum line 32. The fluid 12 is confined by capillary force, as the fluid thickness is typically on the order of about 1–2 mm. Further confinement may be achieved by using vacuum channels and/or air bearing(s) (not shown) at the outer region of the immersion head 26. The fluid used in an immersion optical projection system is typically ultra-pure, deionized water, which provides a refractive index above that of the usual air gap between the lens and the wafer surface. Additives or dopants may be added to the water to give a higher refractive index.
In a fluid immersion system, it is usually preferred to use a fluid with a high refractive index and low absorption. It is undesirable for the fluid to absorb particles from the wafer. However, while using the system 10 of FIG. 1, particles from the photoresist layer still tend to get into the circulated fluid stream 12. Such particles can be carried to the surface of the last lens element 22. This is undesirable because it can contaminate the lens and eventually require a lens replacement, which is expensive. Hence, there is a need for an immersion optical projection system for use in photolithography that will not, or is less likely to, contaminate the lens while still providing the advantages and desirable functions of a fluid immersion system.